Nicolas Kamp

Development of new damage tolerant alloys for age-forming

The suitability of age forming for the shaping of damage tolerant structures is investigated by formulating and testing new alloy-age forming combinations. The alloy formulation process is driven initially by modelling of strength and semi-quantitative understanding of other microstructure-property relations. Using this a range of Al-Cu-Mg-Li-(Zr-Mn) based alloys predicted to provide yield strengths in aged condition comparable with incumbent 2024-T351 alloy for lower wing skins are selected. It is shown that several of these new alloys after artificial aging representative of age-forming have proof strength (PS), fatigue crack growth resistance (FCGR) and toughness that are comparable or better than 2024-T351. UTS to PS ratios of the new alloys are lower than 2024-T351.

Microstructural Modelling for Friction Stir Welding of Aluminium Alloys

Predictive models have been developed to calculate the hardness profile and precipitate distribution after friction stir welding of aerospace aluminium alloys. A coupled modelling approach has been used, linking a thermal model to predict the thermal cycle to microstructure and property models. Two levels of modelling have been considered. The first is a semi-empirical approach that predicts hardness profiles. This simple model is demonstrated to correctly reproduce measured profiles in an example weld. The second model provides a detailed prediction of the precipitate evolution. Particle nucleation, growth, dissolution, and coarsening are all accounted for. Model predictions have been compared with experiments for the case of friction stir welded AA7449 thick plate.

Microstructural Evolution in AA7449 Plate Subject to Friction Stir Welding and Post Weld Heat Treatment

The effect of friction stir welding (FSW) and post weld heat treatment (PWHT) on the second phase particle distribution and cross weld hardness profile in AA7449 plate has been investigated. The alloy was received in an underaged condition, welded, then PWHT to give an overaged condition (in the parent material) . The effect of this complex treatment on the precipitate distribution in the weld and parent plate has been investigated over a range of length scales using small angle X-ray scattering (SAXS), TEM and FEGSEM. It is shown that the PWHT does not improve the hardness in the heat affected zone (HAZ), which is the location of the strength minimum after welding, but it does reduce the difference between the hardness in the HAZ and the nugget and parent hardness. The reduction in nugget strength after PWHT is particularly marked and is due to replacement of fine GP zones formed on post weld natural ageing by coarse overaged precipitates.

Constituent Particles and Dispersoids in an Al-Mn-Fe-Si Alloy Studied in Three-Dimensions by Serial Sectioning

Second phase particles in wrought aluminium alloys are crucial in controlling recrystallization and texture. In Al-Mn-Fe-Si (3xxx) alloys, the size, spacing, and distribution of both large constituent particles and small dispersoids are manipulated by heat treatment to obtain the required final microstructure and texture for operations such as can-making. Understanding how these particles evolve as a function of process conditions is thus critical to optimize alloy performance. In this study, a novel 3-dimensional technique involving serial sectioning in the scanning electron microscope (SEM) has been used to analyse the intermetallic particles found in an as-cast and homogenized Al-Mn-Fe-Si alloy. This has allowed an accurate determination of the size and shape of the constituent particles and dispersoids derived from a 3-dimensional dataset. It is demonstrated that a proper consideration of the 3-dimensional microstructure reveals important features that are not obvious from 2-dimensional sections alone.

Modelling of fracture toughness in high strength 7xxx aluminium alloys

The relationship between plane strain fracture toughness, KIc, tensile properties and the microstructure of two high strength 7xxx alloys has been examined. Two formulations of the 7449 composition were examined, specifically, one containing Zr as the dispersoid forming element, whilst the other contained Mn. Fractography revealed coarse voiding at intermetallics, fine tensile voiding at dispersoids and a combined intergranular/transgranular shear fracture mode. In the Zr containing alloy in particular, the observed increase in toughness with overageing is accompanied by a change in fracture mode from predominantly intergranular/transgranular shear failure to coarse voiding. A new fracture toughness model has been derived based on the microstructurally dependent work hardening factor, KA, introduced in Ashby’s theory of work hardening. This model predicts a linear relationship between KIc and KA^0.85/?ys^0.35 which is shown to be consistent with the experimental data.

The effect of hot deformation on dispersoid evolution in a model 3xxx alloy

The influence of hot deformation on the evolution of size, shape, and fraction of dispersoids has been studied in a simple 3xxx aluminium alloy by means of hot torsion testing. It has been shown that at high strain rates, deformation leads to spheroidization of the dispersoids, an increase in number density, and an increase in volume fraction. The increase in number density and volume fraction are associated with precipitation of new particles. The enhancement of manganese diffusion is a key factor in promoting rapid dispersoid evolution during deformation. A model has been developed to estimate the effect of deformation induced vacancies and dislocations on diffusion. This predicts that an order of magnitude increase in diffusion coefficient between may occur under typical hot deformation conditions, consistent with the rapid microstructural changes measured experimentally.

Modelling precipitate evolution during friction stir welding of aerospace aluminium alloys

Two models to predict the microstructural evolution and post-weld properties of friction stir welds in aerospace aluminium alloys are presented. The first model is a develop- ment of an existing semi-empirical method for the prediction of hardness profiles after welding, calibrated using isothermal hardness data. Post-weld natural ageing is accounted for, and a new method that predicts natural ageing kinetics is introduced. Once calibrated, the model is shown to accurately predict weld hardness profiles. However, this model does not explicitly predict the microstructure and therefore cannot readily be extended to model other properties. It can also only be applied to alloys welded in peak or overaged conditions. The second model aims to explicitly predict the heterogeneous precipitate distributions obtained after welding for any initial condition. It is based on classical kinetic theory and the numerical framework of Kampmann and Wagner. Multiple nucleation sites and multiple phases are accounted for. This model provides detailed microstructural information required for prediction of complex properties.

Effect of Strain and Strain Rate on the Evolution of Dispersoid Particles in Al-Mn-Fe-Si Alloy during Hot Deformation

The development of both dispersoid and constituent particle types during high temperature deformation has been investigated. Using torsion testing, which enables good temperature and strain rate control, the development of particles in terms of individual properties and the overall population has been examined during extended high strain rate deformation. Torsion tests also allow material that has the same thermal history but different levels of strain within a single sample to be compared. Quantitative comparison of particles has been performed using high resolution SEM imaging. Strain has been shown to have an important influence on particle evolution, beyond changing the kinetics of particle evolution alone. It has been demonstrated that the shape of the dispersoids is altered when they are evolving under the action of strain compared to that obtained from a thermal effect.